This investigation aims to elucidate the elastic anatomy of fractures in laboratory rock specimens, using full-field ultrasonic measurements. To this end, a three-step testing paradigm is adopted, which includes:
ultrasonic testing of the intact rock,
fracturing, and
ultrasonic interrogation of the fractured rock.
Experiments are performed on a slab-like prismatic specimen of charcoal granite. The sample is illuminated under the plane stress condition, prior to and post-fracturing, by a shear piezoelectric transducer excited at 10 and 30 kHz. The generated (in-plane) wave motion in the specimen is then monitored over a rectangular region covering the fracture by a 3D scanning laser doppler vibrometer. After suitable signal processing, the full-field ultrasonic waveforms are used to:
reconstruct the curvilinear fracture geometry,
compute the maps of (heterogeneous) elastic modulus in the specimen, using the data interpretation technique known as elastography, and
recover the profiles of (heterogeneous) shear and normal specific stiffness along the fracture.
Fracturing-induced damage in rock and its elastic attributes, namely the reduction of elastic moduli in a local damage zone and heterogeneous contact condition at the fracture interface, are the subject of mounting interest in various facets of geophysical science and technology including energy production from unconventional resources (Baird et al., 2013; Verdon and Wustefeld, 2013; Taron and Elsworth, 2010), environmental protection (Place et al., 2014), seismology (McLaskey et al., 2012), mining (Gu, Morgenstern, and Robertson, 1993), and aseismic failure of rock faults (Calo', Dorbath, and Cornet, 2011). In principle the heterogeneous elastic contact along fractures and faults, which is often driven by a non-uniform stress distribution in the subsurface, can be effectively parametrized in terms of shear and normal specific stiffnesses, relating the contact traction to the jump in displacements across the interface (Schoenberg, 1980). These interfacial parameters – strongly correlated with the surface roughness, geostatic stress, material properties of the bulk rock, and characteristics of the pore/interfacial fluid, if any (Pyrak-Nolte and Morris, 2000) – play a critical role in the stability and strength analysis of rock discontinuities, and control the key characteristics of fracture networks in reservoirs. For example, the inhomogeneous contact condition is responsible for the progressive failure along discontinuities that may occur well before the frictional resistance of the entire interface is surpassed (Hedayat, Pyrak-Nolte, and Bobet, 2014; Eberhardt, Stead, and Coggan, 2004). Moreover, the hydraulic conductivity of subsurface discontinuities is directly related to the elastic nature of their interface (Pyrak-Nolte and Nolte, 2016). Therefore, a proper elastic representation of fractures in continuum models of the subsurface may lead to better understanding of subterranean fluid flow through fractured rock, and thus enhance the (gas/geothermal) reservoir prognosis (Pyrak-Nolte and Nolte, 2016).
